1. Field of Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device or structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Lithographic apparatus comprise a reference frame, which may also be referred to as a metrology frame in the art. The reference frame provides support for the projection system. In certain types of conventional lithographic apparatus, the reference frame is isolated from disturbances caused by other components of the lithographic apparatus, such as the long and short stroke motors used to drive the reticle and wafer stages. Reference frames are conventionally made from materials having a low coefficient of thermal expansion, such as alloys including INVAR™. Until now, it has been assumed that such materials having a low coefficient of thermal expansion are required in order to meet the thermal demands of the reference frame. Unfortunately, these materials are expensive, resulting in a high manufacture cost. Furthermore, such materials are in limited supply and have a limited manufacturability. Operating in a dynamic market of supply and demand, these factors contribute to an unacceptably long lead time for the production of a reference frame. Such a long lead time also includes a large expense in terms of the man-hours required to construct the reference frame due to the sub-optimal manufacturability of conventional reference frame materials. Due to problems in supply and demand of reference frames, it has been found that it is difficult to supply reference frames in sufficiently high volume in a market upturn, thereby losing output, and it is not possible to reduce output volume in a market down-turn, thereby forcing goods to remain in storage.
U.S. Pat. No. 6,529,264 discloses a frame for connecting parts of an optical system, comprising two barrels disposed on top of their own flange-connection to an assembly frame. The patent addresses the problem that movements between certain points of the optical axis that are relatively weakly connected result in imaging performance loss. In particular, this patent addresses the problem of reducing these movements in the frame. It would appear that the frame is made of materials including aluminium and stainless steel. The frame does not constitute a reference frame, but can be seen as part of a projection optics assembly, where it provides the function of improving the imaging performance of this assembly. The frame suffers vibrations, which are transferred to the optical barrel. U.S. Pat. No. 6,529,264 addresses the problem of reducing vibrations which are transferred to the optical barrel. Further, U.S. Pat. No. 6,529,264 merely addresses the dynamical problems in optical systems that are built up from several separate sub-assemblies which much be kept dynamically stable with respect to one another. In U.S. Pat. No. 6,529,264, the required additional solution includes providing an additional frame. The provision of an additional frame produces an overdetermined construction, because a single lower assembly frame, would be sufficient. To overcome the mechanical problems of overdetermination, the solution would be to make the frame a separate part that is only stiff in limited directions and to connect it after the two barrels are positioned to each other through the assembly frame, as proposed in U.S. Pat. No. 6,529,264. Further, the thermo-dynamical problems of an overdetermined assembly would be to make the materials of the frame parts and the assembly frame part the same, also as proposed in U.S. Pat. No. 6,529,264.